In recent years, the development has rapidly progressed in the technology for increasing the output capacity of a laser oscillator, and the technology is expected to be applied to deep-penetration/high-speed welding.
In this environment, the inventor has conducted studies and researches regarding low-heat-input/deep-penetration welding using a high-output-laser as an object of building a strong and tough structure.
The most significant problems arising when achieving the aforementioned object is that the difficulty increases in maintaining a keyhole stable b cause of an increas in the large p netr tion depth, and the possibility of causing defects such as porosities, blowholes, and cracks is thereby increased.
As a result, the inventor obtained knowledges that phenomena occurring at the keyhole and around the keyhole need to be appropriately controlled to prevent the aforementioned defects.
To find a means for implementing the above, the inventor conducted studies and researches regarding influences of a pulse waveform and a ratio between a peak output and a base output on the prevention of the defects. Then, the inventor found that when the flow of molten metal is controlled by introducing modulation to a laser output in accordance with an appropriate frequency and a waveform, the weld defects can significantly be reduced.
For example, as a pulsed-arc welding method employing control of the waveform of output variations, there is a method intended to prevent defects such as blowholes, cracks, and spatter by using a YAG laser. However, since a molten-pool natural oscillation frequency is not used for the frequency of the output variations, the frequency of the output variations does not activate the periodic flow of molten metal and is not sufficient to prevent defects in the de p-p n tration welding.
There is another method int nded to pr v nt porosities that can occur in a zinc-plated steel sheet. However, also in this case, defects occur according to a completely different occurrence mechanism of defects that are caused due to a high vapor pressure of zinc, and the object material and the penetration depth are peculiar.
Accordingly, an object of the present invention is to provide a new laser welding method according to knowledges obtained by the inventor, with which the problems occurring with the conventional techniques are solved, occurrence of melt defects is prevented, a high-quality weld can thereby be provided, and the laser-using deep-penetration welding technique can be used in a wide range of material processes.